Polymer modified hydraulic cement compositions are widely used in industry. There has been extensive investigations of polymer modified hydraulic cement which shows that such polymer modifications of cementitious compositions improve strength, adhesion, flexibility and workability. Much of the modifier admixtures have been in the form of aqueous dispersions. The disadvantage to using a liquid latex is that it requires the end user to mix the liquid polymer composition with the cement at the job site, thereby requiring a two-component mixture. In addition, the two components have to be packaged separately, a wet container for the latex and a dry container or bag for the cement.
The concept of using a redispersible, dry polymer for cement modification is such that the powdered polymer is dry mixed with the cement and aggregate premixture at the job site; all that is required is the addition of water. During the wet mixing with water, the powder polymer becomes re-emulsified, or redispersed, in the modified cement mixture and performs similarly to the liquid latex.
Redispersible polymer powders are polymer emulsions which have been converted by spray drying (atomizing the aqueous polymer dispersions in a stream of hot air) to free flowing powders. When blended with water, these powders redisperse back into liquid emulsions with essentially identical properties to the original emulsions.
With dispersions whose polymers have glass transition temperatures (Tg) below about 50.degree. C., it is necessary to add dispersing aids to coat the individual spray dried particles to prevent them from sticking together and to promote redispersion of the particles into an emulsion. Sticking can occur in the spray drying apparatus when the powder is at elevated temperatures, or upon long term storage under the influences of heat, humidity and pressure.
U.S. Pat. No. 3,409,578 discloses the preparation of a dry cementitious composition modified by an acrylic polymer powder. The acrylic polymer powder is prepared from a latex and contains carboxylate groups in the dispersed polymer, or the dispersing agent for the polymer, or in both. The carboxylate groups may be in the acid form or in the form of a monovalent metal salt or an ammonium salt.
U.S. Pat. No. 3,196,122 discloses cementitious compositions prepared using acrylic polymer emulsions.
U.S. Pat. No. 3,547,853 discloses cementitious compositions comprising a dry redispersible acrylic polymer, an aliphatic hydroxycarboxylic acid set-retarding agent that also serves as a sequestrant, and trimethylol propane, trimethylol ethane or mixture of the latter two compounds. The acrylic polymer may be any water-insoluble emulsion polymer of one or more (meth)acrylic esters or such a polymer containing about 1 to 10 wt % olefinically unsaturated acid.